How to improve "square" artifacts on smoke simulation?
There are a few thing that you need to play with to get the right look.


Make sure your render samples are high enough.
Under the geometry dropdown make sure you change the step size.  





Make sure your divisions are high enough in the domain object. 




I actually created a preset pack (for Cycles) of different smoke elements to get started quicker. I also created a material to control the smoke a little easier. Its not perfect but its good for a lot of scenarios. 

Hope this helps. 

https://dl.dropboxusercontent.com/u/23859364/Pollution.zip